The present invention relates to subterranean well cementing operations, and more particularly, to cement compositions comprising melt-processed inorganic fibers and methods for using such cement compositions.
Hydraulic cement compositions commonly are utilized in subterranean operations, particularly subterranean well completion and remedial operations. For example, hydraulic cement compositions are used in primary cementing operations whereby pipe strings, such as casing and liners, are cemented in well bores. In performing primary cementing operations, usually a hydraulic cement composition is pumped into an annular space between the walls of a well bore and the exterior surface of the pipe string disposed therein. The cement composition sets in the annular space, forming therein an annular sheath of hardened, substantially impermeable cement that supports and positions the pipe string in the well bore and bonds the exterior surface of the pipe string to the walls of the well bore. Hydraulic cement compositions also are used in remedial cementing operations, such as plugging highly permeable zones or fractures in well bores, plugging cracks and holes in pipe strings, and the like.
When the cement compositions contact permeable subterranean formations, fluid (e.g., water) may be lost into the formation. Excessive fluid loss may cause the cement composition to become prematurely dehydrated, thereby potentially causing bridging in the annulus and limiting the time for which said slurry can remain pumpable, and/or reducing bond strength between the set cement composition and a subterranean zone, the walls of pipe and/or the walls of the well bore. Fluid loss control additives (e.g., polymers and copolymers) may be included in a cement composition, inter alia, to reduce fluid loss into the formation. When the permeability of the formation is high, for example, because of unconsolidated or depleted formations, or microfractures, the rate of fluid loss may increase to an extent that some conventional fluid loss control additives (e.g., polymer and copolymers) may not be effective in preventing fluid loss from cement compositions. In an extreme case, fluid loss may increase to the point where the cement composition no longer can be circulated back to the surface—in such case, the cementing operation being conducted may be said to have “lost circulation.” To help control fluid loss, and to prevent fluid loss from escalating to “lost circulation,” certain types of fluid loss control additives that sometimes are referred to as “lost circulation materials” may be included in cement compositions. Examples of conventional lost circulation materials include peanut shells, mica, cellophane, walnut shells, calcium carbonate, plant fibers, cottonseed hulls, ground rubber, and polymeric materials.
Lost circulation also can occur during drilling of subterranean well bores. For example, fluid may be lost into high-permeability zones (e.g., unconsolidated zones or depleted formations), vugular zones, and fractures (e.g., either pre-existing fractures or fractures created during the subterranean operation). Conventional attempts to prevent lost circulation during subterranean drilling operations have involved, for example, the addition of soluble additives (e.g., polymers) to drilling fluids. However, when circulation losses exceed 1 barrel per hour, these additives may not be as effective as desired.
In many cases when circulation losses have been encountered that exceed 1 barrel per hour, conventional insoluble particulate materials (e.g., fibers) have been added to the drilling fluid. Such conventional insoluble particulate materials may form a filter cake on the walls of the well bore. This filter cake may be less permeable than the well bore walls, and, accordingly the establishment of the filter cake may reduce circulation losses. However, the use of conventional insoluble particulate materials may be problematic. For example, if the conventional particulate materials are not chosen carefully, they may cause pumping problems or plug flow lines. In some circumstances, the conventional particulate materials may be screened out on shale shakers (and thus be prevented from remaining with the circulating fluid as it flows into the well bore). In some cases, the comparatively lighter density of the conventional particulate materials may cause them to tend to remain afloat within the comparatively denser drilling fluid. In other circumstances, the addition of conventional particulate materials may cause the drilling fluid to become excessively thick and viscous.